1. Field of the Disclosure
This disclosure relates to a method of manufacturing a liquid crystal display device.
2. Description of the Related Art
Display devices have been largely highlighted in the recent information society as an important visual communication media. The display devices must meet the requirements of having low power consumption, slim size, light weight, high definition, and others, in order to highly occupy a market in future.
Actually, liquid crystal display (LCD) devices corresponding to the main devices among current flat panel display (FPD) devices satisfy not only the requirements of the display device, but also have mass-productivity and improved performance. As such, the LCD devices have been rapidly developed to provide a variety of new functions and positioned on the center of the display devices to gradually replace previous cathode ray tubes (CRTs).
In general, the LCD device supplies to liquid crystal cells arranged in a matrix shape with data signals corresponding to image information and controls the light transmittance of the liquid crystal cells, in order to display a desired image. In other words, the LCD device is mainly driven in an active matrix (AM) system. To this end, the LCD device of an AM system uses an amorphous silicon thin film transistor (a-Si TFTs) as a switch element and drives liquid crystal cells included in a pixel portion. The a-Si TFT can be formed through a low temperature process and makes it possible to use an insulation substrate with a low price. As such, the a-Si TFT has been widely used.
However, since the a-Si TFT has an electric mobility range of 1 Cm2/Vsec, it is difficult to be used in peripheral circuits which operate by frequency signals of at least 1 MHz. To address this matter, integrating methods have been actively developed which simultaneously form a pixel portion and a driving circuit, which are then configured to each include polycrystalline silicon (poly-Si) thin film transistors having a larger field effect mobility than that of the a-Si TFT, on a glass substrate.
The increment of carrier mobility enhances an operating frequency of the driving circuit which determines the number of driving pixels. As such, it is easy to make the definition of a display device higher. Also, the charging time of a signal voltage in the pixel portion becomes shorter so that the distortion of a transmitting signal is reduced. Accordingly, picture-quality can be enhanced. Furthermore, the poly-Si TFT can be driven in a voltage range of below 10V unlike the a-Si TFT having a high driving voltage of about 25V, thereby reducing electric power consumption.
FIG. 1 is a planar view showing schematically the structure of an LCD device according to the related art. A driver-united LCD device including an array substrate integrated with a driving circuit is shown in FIG. 1.
As shown in the drawing, the LCD device is configured to include a color filter substrate 5, an array substrate 10, and a liquid crystal layer (not shown) between the two substrates 5 and 10. The array substrate 10 is configured to include a pixel portion 35 in which unit pixels are arranged in a matrix shape, and a driving circuit portion 30 consisting of a data driver circuit unit 31 and a gate driver circuit unit 32 positioned on outer areas of the pixel portion 35. The pixel portion 25 corresponds to an image display area.
Although they are not shown in the drawing, the pixel portion 35 of the array substrate 10 includes a plurality of gate lines, a plurality of data lines, a plurality of thin film transistors, and a plurality of pixel electrodes. The plurality of gate lines and the plurality of data lines are arranged in horizontal and vertical directions, respectively, thereby defining a plurality of pixel regions. Each of the thin film transistors is formed at an intersection of the gate and data lines and used as a switching element. The plurality of pixel electrodes are formed on the plurality of pixel regions, respectively. Also, each of the thin film transistors is used to switch a signal voltage to be applied to the pixel electrode and to control the flow of an electric current by an electric field.
The driving circuit portion 30 is positioned on the outer areas of the pixel portion 35 of the array substrate 10 which are exposed outwardly from the color filter substrate 5. The data driver circuit unit 31 is disposed on an exposed longer side area of the array substrate 10, and the gate driver circuit unit 32 is disposed on an exposed short side area of the array substrate 10.
The data driver circuit unit 31 and the gate driver circuit unit 32 uses a CMOS (complementary metal oxide semiconductor) thin film transistor corresponding to an inverter, in order to properly output an input signal. The CMOS thin film transistor is an integrated circuit of the MOS configuration which is applied to the driving circuit portion 30 requiring a high-speed signal process. The CMOS thin film transistor is configured to include a P-channel thin film transistor and an N-channel thin film transistor and has interim characteristics between those of the PMOS and NMOS thin film transistors in carrier moving speed and density.
The gate driver circuit unit 32 applies scan signals to the gate lines. The data driver circuit unit 31 applies data signals to the pixel electrodes through the data lines. To this end, the data and gate driver circuit units 31 and 32 are connected to external signal input terminals (not shown). Also, the data and gate driver circuit unit 31 and 32 control external signals from the external signal input terminals and output the controlled signals to the gate lines and the pixel electrodes.
The color filter substrate 5 is configured to include color filters (not shown) and a common electrode (not shown) which are formed on its pixel portion 35. The color filters are used to realize a variety of colors. The common electrode is formed opposite to the pixel electrodes on the array substrate 10.
The color filter substrate 5 and the array substrate 10 are separated at a fixed interval from each other by spacers (not shown) and are provided with a cell gap. The color filter substrate 5 and the array substrate 10 are combined with each other by means of a seal pattern (not shown) which is formed to surround the pixel portion 35, thereby forming a unit LCD panel. A process of combining the two substrates 5 and 10 is performed using at least one combining key formed on the color filter substrate 5 or the array substrate 10.
In this manner, the array substrate 10 of the LCD device is basically configured to include the plurality of thin film transistors. As such, a plurality of masking processes (i.e., photolithography processes) must be performed to manufacture the array substrate 10, thereby deteriorating the productivity of the LCD device. In view of this point, a method capable of reducing the number of masking processes has been required.
More specifically, the photolithography process forms a desired pattern by transcribing the pattern designed on a mask onto a substrate covered with a thin film. To this end, the photolithography process must perform a plurality of steps of photo resist coating, light exposing, developing, and others. Due to this, the production rate of the LCD devices decreases.
Furthermore, the price of the mask used to form the desired pattern is very high. As such, the manufacturing costs of the LCD device increase in proportion to the number of masks which are used in the manufacturing process.